Journal of Geochemical Exploration, 6 (1976) 321--330 321 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

THE DETERMINATION OF ARSENIC AND ANTIMONY IN GEOLOGICAL MATERIALS BY FLAMELESS ATOMIC ABSORPTION SPECTROMETRY

G.E.M. ASLIN

Geological Survey of Canada, Ottawa, Ont. (Canada)

(Received February 9, 1976; revised and accepted March 22, 197.6)

ABSTRACT

Aslin, G.E.M., 1976. The determination of arsenic and ant imony in geological materials by flameless atomic absorption spectrometry. J, Geochem. Explor., 6: 321--330.

The application of flameless atomic absorption spectrometry to the determination of As and Sb in samples used for geochemical exploration is studied.

As and Sb are separated from the sample leach solution via hydride formation with sodium borohydride as reductant. The hydrides are passed into a heated quartz cell where atomic absorption measurement is made. Two acid attacks are investigated and potential interferences examined. Factors affecting sensitivity and precision are discussed in detail. Sixteen international reference samples are taken through the procedure and the values obtained compared with other data. The detection limits, based on 500 mg of digested sample, are 80 ng/g for Sb and 160 ng/g for As. The advantages of the scheme are its sensitivity, speed, versatility, simplicity and apparent freedom from interferences.

INTRODUCTION

Sb and As have historically been determined in geological materials using colorimetric techniques. At the Geological Survey of Canada (G.S.C.), the procedure followed for Sb determination has been that described by Stanton (1966), where, after a 6M HC1 leach or a potassium pyrosulphate fusion, the blue complex formed by reaction of chloroantimonate ions (SbC16 -) with Brilliant Green is measured by visual comparison with standards. Ward et al. (1954), at the U.S. Geological Survey, established a method for determining Sb in rocks and soils by measuring its complex formed with Rhodamine B. This has now been superceded by a faster, more specific method developed by Welsch and Chao (1975), where Sb is measured by atomic absorption in an air-acetylene flame after extraction with TOPO (trioctyl phosphine oxide) into MIBK (methyl isobutyl ketone).

The Gutzeit test for As, as used by Ward et al. (1963), was replaced at the G.S.C. by the silver diethyl dithiocarbamate (AgDDC) colorimetric technique as described by Vasak and Sedivec (1952), and, later, Gastinger (1972). Amine is formed quantitatively by a zinc-acid reduction and the gas absorbed by a brucine-chloroform mixture containing AgDDC, the complex then being measured at 520 nm.

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Hydride generation with subsequent atomic absorption (AA) measurement in a suitable flame (usually argon-hydrogen-entrained air) has become an established method for the determination of these two elements in the last five years. Fernandez (1973) has shown the superiority of sodium borohydride as a reducing agent compared to the zinc-acid combination. Vijan and Wood (1974) designed a system whereby the arsine is swept into a heated, window- less quartz cell for subsequent atomisation. Replacement of the flame by a quartz cell avoids any background absorption by flame gases below 200 nm and possible chemical interference problems, as well as allowing increased residence time in the light path and, hence, higher sensitivity and improved signal stability. Thompson and Thomerson (1974) have outlined a fast, sen- sitive method for determining As, Sb, and six other elements wherein the liberated hydrides are passed directly into a 17-cm silica tube mounted in an air-acetylene flame. They improved upon the speed and sensitivity of analysis attained by Pollock and West (1972) by avoiding the use of a collection vessel for the hydride and by reducing the dilution of the analyte atom population in the light path. The method described in this paper is an adaptation of that by Thompson and Thomerson. Thompson (1975) has recently improved both the linearity of calibration graphs and the detection limits for these elements by measuring atomic fluorescence rather than absorption.

Regardless of this proliferation of analytical techniques based upon hydride generation, little has been published on interference studies until Smith's (1975) extensive investigation. He found that Ni severely suppressed (greater than 50%) both As and Sb signals and that Cu, Co, and Ag moderately interfered (10-- 50% suppression). He cites other elements, such as the platinum metals, as producing severe interference on both elements but it was decided to ignore these in this study as their naturally occurring levels in geological materials would be far below the concentration giving significant suppression. Smith also found mutual interferences of all the volatile hydrides on each other, a phenomenon caused by the cool argon-hydrogen flame. Again, this would not be a problem when the atomisation cell is a silica tube heated by a hot air-acetylene flame. Iron was also investigated by the author as Braman et al. (1972) found it to hinder the generation of stibine.

Two types of decomposition procedures were studied and compared; hot aqua regia and the common HF-HC104-HNO3 attack used for the determination of most trace elements.

EXPERIMENTAL

Reagents

Stock solutions containing 1000 ~g/ml of As(II1) and As(V) were prepared by dissolving Analar grade As~O3 and Na3AsO4 in deionised water with the required amount of NaOH and then acidifying with HC1. A stock solution con- taining 1000 pg/ml of Sb(III) was prepared by dissolving Fisher "certified"

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ant imony potassium tartrate in 6M HC1. All diluted solutions were prepared daily.

Fisher reference standard solutions of Fe, Co, Ni, Cu and Ag at the 1000 pg/ ml level were used for the interference studies.

A 1--4% m/V aqueous solution of J.T. Baker sodium borohydride was fresh- ly prepared for each set of measurements.

Hydrofluoric, hydrochloric, perchloric and nitric acids were all "Baker analysed" reagent grade.

Instrumentation

The experimental arrangement is depicted in Fig. 1. Cell A is simply a 18 mm X 116 mm plastic test~tube with a hole drilled in it 30 mm from the bo t tom to accommodate the tip of an Eppendorf pipette. A ul aliquot of the acidified sample solution is injected into the cell containing 2 ml of 1--4% sodium borohydride solution and the hydride(s) generated is swept by a con- stant f low of nitrogen (B) into the heated quartz tube D rigidly positioned 25 mm above a triple slot air-acetylene burner. The flow of nitrogen (C) at either end of tube D is merely to prevent combust ion of excess hydrogen formed in the reduction process.

_~J [ silica tube ] ~

air coolant

nitrogen B

plastic stopper (tight fit)

cell A

- - t y ~

t L ~ r ~

_ • _ _ Eppendor f tip

sodium -- borohydride

solution

Fig. 1. Hydride generator and atomising cell.

A Perkin-Elmer 303 atomic absorption spect rophotometer was used with background correction. The light sources were As and Sb electrodeless dis- charge lamps manufactured by Perkin-Elmer. A spectral band pass of 0.2 nm was used for each element. As was measured at 193.7 nm and Sb at 217.6 nm. Peak height absorbances were read on a Coleman 56 chart recorder in series

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with a solid state computer interface designed and manufactured "in-house" by Bristow (1974).

Procedure

Optimisation of sensitivity and precision. A 10.0 ng/ml solution of each element was used to optimise the following parameters: (1) nitrogen (B) flow- rate, (2) strength and volume of sodium borohydride, (3) acidity of sample solution, and (4) aliquot of sample solution to be injected into cell A.

Sample decomposition. (1) Aqua regia. 10.0 ml of freshly prepared aqua regia was added to 250 mg of the sample in a calibrated test tube. The solution was allowed to sit in the cold for 11/i hours and, subsequently, in a water bath at 90°C for 2 hours. A Vortex mixer was used three times during this decomposi- tion period to ensure thorough attack by the acid mixture. After cooling, the volume was made up to 10 ml with aqua regia, the solution mixed and allowed to settle. An aliquot of 1000 ~l or less of this solution was taken and diluted to 10.0 ml with 0.5M HC1. A pl aliquot of this dilute sample solution was then injected into cell A containing the reducing agent, sodium borohydride. The arsine or stibine liberated was measured by the peak absorbance traced on the recorder. This cell was then replaced by another and the next sample analysed. Standard solutions having a final concentration range of 0.0--50.0 ng/ml were also taken through this procedure.

(2) HF-HC104-HNO3.7 ml of 50% HF were added to 250 mg of the sample in a platinum dish situated on a hot plate. The sample was gradually taken to dryness, after which 5 ml of HNO3 and 2 ml of HC104 were added. The solu- tion was evaporated to white fumes of perchloric acid. The sample was then taken up in 5 ml of 2M HC1, warmed and transfered to a calibrated test-tube, where, after cooling, the solution was made up to 10.0 ml with deionised water An aliquot of 1000/~1 or less of this solution was diluted up to 10.0 ml with 1.5M HC1, ready for analysis. Again, standard solutions of As and Sb were subjected to this procedure.

Interference study. Standard solutions, containing 0.0--50.0 ng/ml of each element in a 10% aqua regia/90% 0.5M HC1 matrix, were spiked with each interferent (Fe, Co, Ni, Cu, Ag) at four different concentration levels: 100, 500, 1000, and 5000 ng/ml. A standard (with no interferent present) was analysed and the signal recorded. This was followed by the analysis of four standards with the interferent present at the four levels chosen and then the original standard rerun. This scheme avoided the possibility of any drift in the instrumentation being misinterpreted as interference.

A standard addition technique was also applied to various sample solutions to check for possible interferences.

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RESULTS AND DISCUSSION

Optimum conditions

A variation in the carrier gas flow rate from 1.0 to 3.0 1/min did not alter the sensitivity for either element by more than 10% bu t precision was found to be opt imum at a flow rate of 2.0 1/min.

A sample aliquot of 500 pl proved superior for injection into the cell. As peak height is the parameter being measured, variation in the rate at which the sample is added to the borohydride will affect results. This source of operator error was minimal at 500 pl.

A combinat ion of a 1.5M sample acidity with 2 ml of 2% sodium borohydride was chosen for the best results in sensitivity and precision. A 1% borohydride concentration was considered too low to ensure complete reduction of As or Sb when other consuming reagents were present, whereas a 4% solution produced too much effervescence and, consequently, fast, erratic signals. Similarly, too much acid in the sample solution created a large excess of hydrogen which produced imprecise and lower peak heights.

The calibration graphs obtained for both elements under these opt imum con- ditions are shown in Fig. 2a and b. The growth curve for As(V) in 1.5M HC1 is identical to that of As(III) made up in 10% aqua regia. Thus the two graphs for each element represent the two valency states prior to reduction. The sig- nal obtained from As(III) in 1.5M HC1 is faster, greater and somewhat less precise than that from As(III) which has been oxidised to As(V) in the aqua regia medium. This difference in signals due to valency states would be elim- inated if integrated absorbance was the parameter being measured. The detec- tion limit, defined as that concentrat ion giving a signal equal to twice the standard deviation of the blank, was 0.8 ng]ml for As and 0.4 ng/ml for Sb; the limit for As was somewhat impaired by the presence of that element as an impuri ty in the sodium borohydride. The relative standard deviation, based on twelve measurements, of a 10.0-ng/ml solution of As and Sb in the aqua regia medium was 3.5% and 3.1%, respectively.

International reference samples

The results of analyses of sixteen international reference samples are given in Table I. Standard solutions taken through both HF-I-IC104-I-INO3 and aqua regia procedures produced calibration graphs identical to that given by stan- dards made up in 10% aqua regia in the cold. Hence, no loss of either element has occurred, contrary to the findings of other chemists (Dolezal et al., 1968). Results by both decomposit ions agree well with each other and with the "usuable" values quoted by Abbey (1975), Abbey et al. (1975), Flanagan (1973), and Allcott and Lakin (1975).

The fact that As or Sb did no t volatilise from the standard solutions (in 1M HNO3 ) when under attack by HF seems somewhat perplexing. However,

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..... i! (a) ...... ' j

, . t

: ! ~ (i) A~) in 10% oqua regia -

n2k_ f [i]) AS{~) inlbM HCI

Concentratlon/ng ml I

Cor~centra*ion / ng ml~1

Fig. 2. (a) Calibration graphs for arsenic. (b) Calibration graphs for antimony.

the good recovery obtained from this decomposit ion on the rocks and soils indicated is not surprising when it is considered how those elements exist in nature (Boyle and Jonasson, 1973). The primary As minerals are sulphides and oxygenated complexes, the element also being found in a great variety of secondary oxidation products, particularly in sulphates. Hydrofluoric acid does not at tack sulphides, such as arsenopyrite, so that the operative stages in leaching out the As (or Sb) would be the HC104-HNO3 addition. The ele- ments would then be oxidised to their stable oxygenated (V) state.

A precision s tudy was carried out on samples MRG-1, SY-2 and SY-3. Seven decomposit ions of three types were made over a period of several months. The results can be seen in Table II. The attack with 10 ml of 6M HC1 was carried out in a test-tube placed in a water bath at 90°C for two hours. The usual procedure is to dilute 1.0 ml of the leach solution to 10.0 ml with the appropriate HCI concentration for samples containing up to 20 ppm As or

327

TABLE I

Results of analyses of international reference samples (values in ~g/g)

Sample As As As Sb Sb Sb aqua regia HF-HCIO4-HNO3 other aqua regia HF-HC104-HNO 3 other

W-1 2.06 2.10 1.9 a 1.00 1.04 G-1 0.70 0.75 0.57 b 0.20 0.20 DTS-1 <0.40 <0.40 0.507 a 0.42 0.46 BCR-1 0.64 0.72 0.70? b 0.60 0.54 GSP-1 <0.40 <0.40 0.097 b 2.80 2.48 AGV-1 0.75 0.78 0.8? b 4.20 4.12 PCC-1 <0.40 <0.40 0.05 b 1.22 1.32 GM 4.20 4.04 4 b 0.51 0.48 BM 13.80 13.50 14 b 1.76 1.66 SU-1 430 415 418 b <0.20 <0.20 SY-2 17.80 18.00 18? d 0.20 0.20 SY-3 19.70 19.85 217 d 0.20 0.20 MRG-1 0.46 0.50 27 d 0.38 0.39 GXR-1 349 335 3007 c 115 122 GXR-2 16.25 16.96 20? c 41.04 40.00 GXR-6 304 310 300? c 3.92 3.98

1.0 a 0.31 b 0.6? a 0.69b 3.1 b 4.3? a 1.4 a 0.5 b 2 b 2.1 b 0.2 e 0.3 e 0.3 e

120? c 40? c

4.6? c

aAbbey (1975) bFlanagan (1973) CAlcott and Lakin (1975) dAbbey et al. (1975) eprivate communication from D.R. Norton, U.S.G.S., to S. Abbey, G.S.C., 1975. Method of

Ward and Lakin (1954) ?Indicated uncertainty due to lack of reliable data.

TABLE II

Precision study of arsenic determinations (As in ug/g)

Sample Weight taken Aqua regia HF-HC104-HNO 3 6M HCI (mg)

SY-2 100 17.80-+0.74 18.00-+0.80 16.94_+0.76 SY-3 100 19.70-+ 0.82 19.85+ 0.84 19.26 + - 0.80 MRG-1 500 0.46-+0.05 0.50-+0.05 0.35-+0.10

Sb (equivalent to 50 ppb in solution). For greater concentrations, a smaller aliquot of leach solution is used or a second dilution made, taking care to maintain the same final acid composition as exists with the standards.

328

(i~ As only

,400 (ii} As ÷ 1 0 0 0 p ~ Ni

/ / IH) ,

.... ! .// ' J

i '

, ) 1 L I I I _ ~ ,'o ~'o 2o - 20 ~o ooo

C o n c e n t r a t l o n / ng ml -I

C o n c e n t r o t i c n / ng ml I

.40

.35 {c~

.30

.75 ( i i ) ,

i .2C (iii~

i

05 / ~ '~-4 ' f h , ! Sb ~ 5 0 0 0 p p b Ag

~ / ' J ~ - ' J ,~v'sb.5o®oob.g ! . ~ ~ ~ , ~ ; ~ j ~: 1o 20 30 I ao 50

Concen t ra t i on , ng ml

Fig. 3. (a) Nickel interference on arsenic. (b) Nickel interference on ant imony . (e) Silver interference on ant imony.

329

Interferences

Fe, Co, and Cu, at the four levels studied, did not interfere with the deter- mination of either element. However, Ni, at the 1000-ppb level, did suppress the As curve slightly and, to a very significant extent, at the 5000-ppb level as indicated in Fig. 3a. Both Ni and Ag interfere in the determination of Sb as shown in Fig. 3b, c. This suppression is thought to be caused by the for- mation of a metal precipitate that absorbs or captures a fraction of the stibine or amine in the reduction. Thus, if a chelating agent is present that forms a strong complex with Ni or Ag and prevents any precipitation, this type of interference would be eliminated. It was found that standard As and Sb solu- tions in the 10% aqua regia matrix containing 5000 ppb Ni and 0.01M EDTA exhibited no suppression whatsoever by the presence of Ni. Fig. 3c shows the diminished interference by 5000 ppb Ag on Sb in the presence of 0.01M EDT& There is no significant interference at the 1000-ppb Ag level in the presence of 0.01M EDTA. Since 5000 ppb Ag is equivalent to 2000 pg/g Ag in the solid (based on 250 mg weight), this interference would be irrelevant for most geo- chemical samples. An example of Ni suppression on Sb determinations can be seen by examining the results obtained on SU-1 by this procedure (Table I). This ore contains about 13,000 ug/g Ni which, according to the above scheme, would result in a final solution concentration of 32,500 ppb and, hence, com- plete suppression of the Sb signal. However, by taking an initial weight of 100 mg (final Ni concentration of 13,000 ppb) and adding the EDTA, a value of 1.90/~g/g Sb was obtained which agrees well with that qt" ~d by Flanagan. It should be noted that the value obtained for As on SU-1 is acceptable and shows no interference by Ni because the dilution factor is necessarily large (20,000).

Both standard addition and direct calibration produced identical results for samples BCR-1, GM, BM, GXR-1, GXR-2 and GXR-6.

CONCLUSIONS

The hydride generation technique, using a sodium borohydride reduction with subsequent detection by atomic absorption, is a sensitive, efficient method for the determination of arsenic and antimony in geological materials. The detection limit, based on a 500 mg weight, is 0.08 #g/g for antimony and 0.16 pg/g for arsenic. The precision of the method is subject to the skill of the operator and would be improved by automation. Fifty samples per day can be analysed for both elements.

This method of analysis is currently being extended to include the deter- mination of selenium, tellurium and tin in solid samples and waters.

ACKNOWLEDGEMENTS

The author wished to thank Mr. Gilles Gauthier for his help in setting up the instrumentation. I also wish to acknowledge the expertise of Mr. J. van den Hoff, National Research Council, in the art of glassblowing.

330

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Abbey, S., 1975. Studies in "Standard Samples" of silicate rocks and minerals, 4. 1974 edition of "usuable" values. Geol. Surv. Can., Paper, 74-41: 19--22.

Abbey, S., Gillieson, A.H. and Perrault, G., 1975. A report on the collaborative analysis of three Canadian rock samples for use as certified reference materials. Canmet Rep., MRP/MSL 75-132 (TR), 56 pp.

Allcott, G.H. and Lakin, H.W., 1975. The homogeneity of six geochemical exploration reference samples. In: I.L. Elliott and W.K. Fletcher (Editors), Geochemical Exploration 1974. Elsevier, Amsterdam, pp. 659--681.

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Bristow, Q., 1974. Solid state computer interface and update unit for existing Perkin-Elmer double beam atomic absorption spectrophotometers. Anal. Chem., 46: 2246--2249.

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Gastinger, E., 1972. The spectrophotometr ic determination of arsenic with silver diethyl- dithiocarbamate. Mikrochim. Acta, 4: 526--543.

Pollock, E.N. and West S.J., 1972. The determination of ant imony at submicrogram levels by atomic absorption spectrophotometry. Atom. Absorption N ewsl., 11: 104--106.

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